WDPN33 PGTW 251500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.8N 150.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 579 NM EAST-SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
11W (NEPARTAK) IS A LARGE SUBTROPICAL CYCLONE THAT REMAINS HIGHLY
ELONGATED ALONG A SOUTH-SOUTHWEST TO NORTH-NORTHEAST AXIS.
ASSIGNING AN ACCURATE CENTER POSITION REMAINS CHALLENGING, AS TWO
PRIMARY AREAS OF ROTATION EXIST ABOUT 350 NM APART WITHIN THE
ELONGATED CIRCULATION. WE ARE CONTINUING TO TRACK THE MIDPOINT
BETWEEN THESE TWO FEATURES AS IT MOVES GRADUALLY NORTHWARD.
NUMERICAL MODEL GUIDANCE IS IN EXCELLENT AGREEMENT THAT THE
CIRCULATION WILL CONSOLIDATE TOWARD THE NORTHERN SIDE DURING THE
NEXT 24 HOURS, AT WHICH POINT A MORE WELL-DEFINED CENTER LOCATION
WILL HOPEFULLY BE IDENTIFIABLE. CONVECTION REMAINS DISORGANIZED AND
CONFINED MOSTLY TO THE EASTERN SIDE OF THE CIRCULATION. DEEP-LAYER
MOISTURE IS BEGINNING TO WRAP AROUND THE NORTHERN SIDE AS THE
LOW-LEVEL CENTER BEGINS TO MOVE AROUND THE NORTHEASTERN SIDE OF A
STRONG UPPER-LEVEL TROUGH OBSERVED IN ANIMATED WATER VAPOR
SATELLITE IMAGERY. DRY AIR ASSOCIATED WITH THIS TROUGH CONTINUES TO
DOMINATE THE WESTERN SIDE OF NEPARTAK'S SURFACE CIRCULATION. THE
INITIAL INTENSITY IS HELD AT 35 KT PRIMARILY BASED ON EARLIER
SCATTEROMETER DATA, SUPPORTED BY PGTW DVORAK ESTIMATE OF T2.5 (35
KT).

INITIAL WIND RADII BASIS: COMBINATION OF SCATTEROMETER DATA AND
NUMERICAL MODEL ANALYSES

CURRENT STEERING MECHANISM: UPPER-LEVEL TROUGH TO THE SOUTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS ADT: 35 KTS AT 251140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EASTWARD
   OTHER FACTORS: DRY AIR IN WESTERN AND SOUTHERN SEMICIRCLES

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: THE MESSY, LARGE CIRCULATION OF 11W (NEPARTAK)
IS
CURRENTLY INTERACTING STRONGLY WITH THE UPPER-LEVEL TROUGH TO ITS
SOUTHWEST. THIS INTERACTION IS BAROCLINIC IN NATURE, AND COULD
INDUCE A SLIGHT INCREASE IN MAXIMUM SURFACE WINDS DESPITE AN
ENVIRONMENT THAT WOULD BE HOSTILE TO A TRULY TROPICAL CYCLONE, 
CHARACTERIZED BY HIGH VERTICAL SHEAR AND NEARBY DRY AIR. THE 
EVOLUTION OF THE CYCLONE DURING THE NEXT 48 HOURS WILL BE SIMILAR TO 
THAT OF MID-LATITUDE OCCLUSION AS 11W (NEPARTAK) HOOKS TO THE 
NORTHWEST AROUND THE UPPER-LEVEL TROUGH AND THEN BECOMES VERTICALLY 
ALIGNED BENEATH IT WHILE APPROACHING JAPAN. MAXIMUM WINDS ARE 
FORECAST TO INCREASE SLIGHTLY TO 40 KTS BY 24 HOURS. ONCE ALIGNMENT
OCCURS, THE CYCLONE WILL BECOME SEPARATED FROM THE MONSOONAL FLOW IN 
WHICH IT IS CURRENTLY EMBEDDED, AND THE WIND FIELD IS FORECAST TO 
SHRINK IN SIZE. DRY AIR ASSOCIATED WITH THE UPPER-LEVEL TROUGH
MAY ENVELOPE THE CIRCULATION AT THIS POINT, AND THE CIRCULATION
CENTER IS FORECAST TO PASS JUST NORTH OF THE KURUSHIO CURRENT OVER
COOLER SEA SURFACE TEMPERATURES (SST) OF LESS THAN 26 DEGREES
CELSIUS BEFORE REACHING JAPAN. SLIGHT WEAKENING TO 35 KT IS THUS
FORECAST AS THE CYCLONE NEARS HONSHU IN AROUND 48 HOURS. NEPARTAK
IS EXPECTED TO BE STEERED NORTHWESTWARD ACROSS NORTHERN HONSHU BY A
WEAK RIDGE OVER KYUSHU. AS THE CYCLONE EMERGES IN THE SEA OF JAPAN,
A SLOWING OF FORWARD MOTION IS EXPECTED AS THE STEERING RIDGE OVER
KYUSHU WEAKENS, AND NEPARTAK IS FORECAST TO BE NEARLY STATIONARY BY
120 HOURS. ANOMALOUSLY WARM WATER WITH SST OF 27-28 DEGREES CELSIUS
IN THE SEA OF JAPAN COULD SUPPORT TRANSITION OF NEPARTAK INTO A
FULLY TROPICAL CYCLONE BY 72-96 HOURS. THEREAFTER, THE CYCLONE MAY
DRIFT NORTHWARD INTO A MORE HOSTILE ENVIRONMENT OF COOLER WATER AND
HIGHER WIND SHEAR BY 120 HOURS. THERE IS LOWER THAN AVERAGE
CONFIDENCE IN THE INTENSITY FORECAST BEYOND 48 HOURS, AS THERE IS
UNCERTAINTY IN WHAT THE STRUCTURE OF THE CYCLONE WILL BE AT THAT
TIME. THE JTWC TRACK FORECAST IS SIMILAR TO THE PREVIOUS ONE.

MODEL DISCUSSION: THE JTWC INTENSITY FORECAST IS SLIGHTLY BELOW THE
DYNAMICAL MODEL CONSENSUS FOR THE FIRST 48 HOURS DUE TO CURRENT
ORGANIZATIONAL TRENDS. THE TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS, WHICH HAS BEEN GENERALLY STEADY DURING THE
LAST 12 HOURS.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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